1. Field of the Invention
The invention concerns a process for the production of a carbonaceous paste based on at least one carbonaceous aggregate and a cokable organic binder. It essentially concerns adjusting the temperature of the paste to the particular conditions involved in shaping thereof as soon as it issues from the mixer.
2. Description of Related Art
Carbonaceous agglomerates are obtained by firing pieces which are shaped from a carbonaceous paste resulting from mixing of a cokable organic binder and a carbonaceous aggregate in the form of grains of graded sizes. Depending on the use to which the agglomerates are to be put, the nature of the binder (coal tar, petroleum tar, liquid or solid tar) and that of the carbonaceous grains (coal coke, petroleum coke, anthracite, etc.) may vary substantially but in all cases the procedure involves the stage of prolonged mixing between the binder and the carbonaceous grains (the granulometric distribution of which is carefully controlled), at a temperature such that the binder is sufficiently fluid (for example 140.degree. C. to 180.degree. C.) and for a period of time which ensures as perfect impregnation as possible by the binder of the carbonaceous grains. The quality of the electrodes (which is evaluated in particular by measuring geometrical density, electrical resistivity and resistance to crushing) after firing is closely linked to the effectiveness of the mixing operation.
In modern works for the production of carbonaceous pastes, as is the case in particular with the manufacture of anodes for the production of aluminum using the HALL-HEROULT process for the electrolysis of alumina in molten cryolite, the operation of mixing the blend of binder plus carbonaceous grains is carried out in a continuous mixing chain which comprises one or sometimes two mixers in series and the temperature of which is regulated at between 140.degree. and 160.degree. C. and which feeds a shaping installation using vibro-compacting.
One type of mixer which is currently in use is formed by a tubular body provided with fixed teeth which are inclined with respect to the axis of the tube, within which a shaft is moved with a reciprocating movement synchronized with a rotational movement, the shaft itself being provided with teeth co-operating with the fixed teeth in order to ensure that the carbonaceous paste is kneaded or worked and caused to flow along the arrangement. The fixed teeth are disposed along a helicoidal line and the amplitude of the backward and forward movement of the shaft is adjusted to the pitch at which the fixed teeth are disposed. The outlet of the mixer or mixers comprises a nozzle which is closed by motorised flap valves, while opening and closing of the valves can be regulated in dependence on thresholds in respect of instantaneous power in order to provide for satisfactory mixing of the paste and to prevent "jamming" or "stalling" of the apparatus, that is to say blocking thereof under load as a result of excessive filling thereof.
That type of mixer was described in particular in CH-A-515 061, CH No. 606 498 and FR-A-2 038 173 in the name of BUSS A.G. A process for regulating the mixing power thereof was described in European patent application EP-A-157 987 in the name of Aluminium Pechiney.
Mention may also be made of a second type of continuous mixer which is also very widely used, comprising two parallel agitators which rotate in opposite directions and which are disposed in side-by-side relationship in a casing of suitable shape. The main shaft carries radial discs which are themselves connected by mixing bars. In parallel to the shaft, the cleaning shaft which carries cleaning frame structures rotates at a ratio in respect of speed which is from 4 to 6 times higher. One or more retaining plates as well as one or more braking blades which are disposed on the body or the cover of the mixer, at the side of the cleaning shaft, make it possible to control the flow of paste within the apparatus. A mixer of that type was described in French patent application FR-A-2 039 628=U.S. Pat. No. 3,687,628 in the name of H. LIST.
The man skilled in the art is aware from experience that in certain situations, for example that which involves shaping the anodes for Hall-Heroult electrolysis tanks, the optimum mixing and shaping temperatures do not coincide.
In order to provide for good penetration of the tar into the grains of coke and good homogeneity in respect of the carbonaceous paste, mixing must be carried out at about 160.degree. C. while the shaping operation, for example using vibro-compacting, has to be performed at a substantially lower temperature (110.degree. to 140.degree. C.), as otherwise the agglomerates collapse far beyond the accepted dimensional tolerances after removal thereof from the mould and prior to the firing operation. It is therefore necessary for the mixed paste to be cooled by one to several tens of degrees prior to the shaping operation.
FR-2 154 842 (=U.S. Pat. No. 3,885,983) in the name of Aluminium Pechiney provides arranging two mixers in series. The paste which issues from the first mixer at a temperature of 150.degree./170.degree. falls under the force of gravity into a vertical column which opens to the intake of the second mixer, the column being provided with controlled water spray means which effect controlled cooling of the paste to about 110.degree.-115.degree. C.
The disadvantage of that process is that it requires the use of two mixers and that the second mixer which operates on the cooled paste requires a much higher level of power by virtue of the increased viscosity of the carbonaceous paste when it has been cooled in that way.